This application claims priority to Japanese Patent Application JP 2000-170431, and the disclosure of that application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a battery pack and is applicable to a battery pack based on a lithium-ion secondary battery, for instance.
2. Description of the Related Art
Conventionally, a battery pack based on a lithium-ion secondary battery is adapted to prevent over-voltage charge and under-voltage discharge by use of a control IC for controlling operations of switching field effect transistors.
FIG. 6 is a connection diagram showing a battery pack. That is, a battery pack 1 has a secondary battery cell 2 and a protective circuit 3 respectively housed in a predetermined case. The battery pack 1, when mounted to a charging device or a loading device, enables charge and discharge currents to be supplied and outputted between the charging device or the loading device and the secondary battery cell 2 through a positive external terminal 4A and a negative external terminal 4B.
In the battery pack 1, a terminal voltage of the secondary battery cell 2 and terminal voltage between the positive external terminal 4A and the negative external terminal 4B or the like are monitored by use of the control IC 5 to permit switching field effect transistors 6, 7 placed in a charge and discharge path to be on-off controlled according to the monitoring results. That is, the battery pack 1 is structured that discharge and charge-control N-channel field effect transistors 6, 7 are placed in series in the charge and discharge path between the negative external terminal 4B and a negative terminal of the secondary battery cell 2. Incidentally, parasitic diode is existent between a source and a drain of each of the N-channel field effect transistors 6, 7 for the structural reasons. Therefore, when the terminal voltage of the secondary battery cell 2 is reduced down to a predetermined value or less, the battery pack 1 switches over the discharge-control field effect transistor 6 to the Off-state to prevent under-voltage discharge. On the other hand, when the terminal voltage of the secondary battery cell 2 is increased up to a predetermined value or more, the battery pack switches over the charge-control field effect transistor 7 to the Off-state to prevent over-voltage charge.
Incidentally, the battery pack 1 applies P-channel field effect transistors 8, 9, instead of the N-channel field effect transistors 6, 7, to constitute the switching means in some cases as shown in FIG. 7.
When high charge and discharge currents are required, the battery pack 31 is structured that the field effect transistors constituting the switching means are connected in parallel to control charge and discharge currents as shown in FIG. 8 by contrast with FIG. 6. That is, the battery pack 31 is provided to supply a control signal outputted from the control IC to gates of the field effect transistors 6A, 6B through a resistor 10. Incidentally, FIG. 8 shows only the discharge-control field effect transistors 6A, 6B without a description of the charge-control field effect transistors. It is to be understood that output impedance of a control signal output terminal in the control IC is considered to be ordinaryly 10 [Kxcexa9] or more, which corresponds to an equivalent circuit having the resistor 10 connected in series.
Incidentally, a user sometimes carries the battery pack of this kind in one""s hand in use, and as a result, high voltage caused by static electricity is applied to the battery pack on such occasions. While the high voltage caused by the static electricity is limited to about 6 to 15 [kV], application of voltage of several [kV] or more is considered to be enough to cause breakdown of the field effect transistors. Accordingly, it is feared that breakdown of the field effect transistors might be caused by static electricity when the user frequently carries the battery pack in one hand in use.
With the breakdown of the field effect transistors caused by the static electricity or the like in the conventional battery pack, a source-to-drain resistance value of the field effect transistor is increased, resulting in difficulty in using the battery pack structured that each of the charge and discharge-control field effect transistors constituting the switching means is placed individually in the charge and discharge path as described in FIGS. 6 and 7. In this connection, while the source-to-drain resistance value is limited to 100 [mxcexa9] or less in a ordinary condition, while being increased up to 1 [kxcexa9] or more in consequence of the breakdown as described the above.
On the other hand, in the battery pack structured that the field effect transistors are connected in parallel as described in FIG. 8, the breakdown of only one of the parallel connected field effect transistors is supposed to be caused by static electricity. In this case, when a large number of field effect transistors are connected in parallel and so on, each field effect transistor makes sure of a capacity enough to permit the remaining field effect transistors to apply sufficient charge and discharge currents in some cases. The battery pack, if made available for such a case, is considered to be convenient. However, the conventional battery pack presents a problem in difficulty in making the battery pack available for such a case.
A description will now be given by taking the case of the battery pack having the structure shown in FIG. 8. That is, the control IC is provided to set the discharge-control field effect transistors 6A, 6B to the On-state and the Off-state on the basis of the rise and fall of gate control voltage of the discharge-control field effect transistors 6A, 6B. The voltage required for setting the discharge-control field effect transistors to the On-state is set at a value approximately equal to the terminal voltage of the secondary battery cell 2, for instance. On the other hand, there is a need for setting the gate control voltage at approximately 0 [V] to set the discharge-control field effect transistors to the Off-state. When the terminal voltage of the secondary battery cell is reduced down to 2 [V], the control IC for use in the lithium-ion secondary battery switches over the field effect transistors 6A, 6B from the On-state to the Off-state.
The resistor 10 in the battery pack is set to have a resistance value of about 100 [kxcexa9] so that a gate-to-source resistance value in each of the field effect transistors 6A, 6B comes to about 1 to 200 [Mxcexa9] in a ordinary condition. Thus, the control IC 5 makes it possible to set the terminal voltage of the control terminals at 4 [V] and 0 [V] for setting the gate voltage of the field effect transistors 6A, 6B at 4 [V] and 0 [V] respectively.
The least gate-to-source voltage required for maintaining the source-to-drain resistance value of each of the field effect transistors 6A, 6B smaller is about 1.5 [V]. Accordingly, the battery pack makes it possible to set the terminal voltage of the control terminals at 4 [V] and 0 [V] for setting the field effect transistors 6A, 6B to the On-state and the Off-state.
On the other hand, when the breakdown of the field effect transistors is caused by static electricity or the like, the gate-to-source resistance of the field effect transistor is reduced down to about 1 [kxcexa9]. Assuming that the breakdown of the field effect transistor 6A is caused by static electricity, for instance, the gate-to-source voltage in the undamaged-side field effect transistor 6B is also reduced down to about 0 [V], resulting in difficulty in setting the field effect transistor 6B to the On-state. Incidentally, FIG. 9 shows a resistance value of each part in the constitution of the battery pack shown in FIG. 8 without a description of the charge-control field effect transistors. In FIG. 9, the source-to-drain resistance value of the field effect transistor is given as the total resistance value of the two field effect transistors 6A, 6B. According to the table in FIG. 9, since the total source-to-drain resistance value of the field effect transistors after the breakdown by static electricity reaches 2 [kxcexa9] even if the terminal voltage of the secondary battery cell 2 is set at 4 [V], it is to be understood that supplied discharge current is limited to 2 [mA](4 [V]÷2 [kxcexa9]) regardless of short-circuiting of a load. For that reasons, when the breakdown of one of the field effect transistors is caused by the static electricity, the battery pack permits no supply of discharge current as much as 3 [mA] to 10 [A], which is considered to be the discharge current in the ordinary condition.
The present invention is made by considering above described points. Accordingly, it would be desired to provide a battery pack having a function of controlling charge and discharge currents by use of parallel-connected field effect transistors, even if breakdown of a part of the field effect transistors is caused, for example, by static electricity.
According to one embodiment of the present invention, there is provided a battery pack having a function of controlling charge current and discharge current by use of parallel-connected field effect transistors constituting charge or discharge-control switching means, available by supplying control voltage to gates of the parallel-connected field effect transistors through resistors of 10 [kxcexa9] or more, even if breakdown of a part of the field effect transistors is caused by static electricity or the like.
In order to attain the above object, a battery pack according to the present invention takes measures to supply control voltage to gates of a plurality of field effect transistors through resistors of 10 [kxcexa9] or more.
Accordingly, even if breakdown of any of the field effect transistors is caused by static electricity or the like and the gate-to-source resistance is reduced to an extremely small value, the battery pack constituted to supply the control voltage to the gates of the plurality of field effect transistors through the resistors of 10 [kxcexa9] or more makes it possible to prevent a reduction of other field effect transistor gate voltage, permitting control of charge and discharge currents by use of the other field effect transistors.